The gp70 of a helper-independent strain of a Friend MCF virus has been purified and radioimmunoassays have been developed which detect specific immunological determinant(s) contained in the MCF viruses derived from Friend, Moloney and AKR MuLV's. This MCF determinant(s) is not detected in the ecotropic parents of any of these MCF viruses, nor in helper-independent murine xenotropic viruses derived from Swiss or BALB/c mice. A protein partially cross-reactive with the MCF gp70 is detected in a replicating xenotropic virus derived from NZB mice. Utilizing this MCF specific immunoassay, a cross-reacting gene product coded for by the Friend strain of the spleen focus-forming virus (SFFV) can be detected, supporting earlier molecular hybridization studies which indicated that the genome of SFFV contains a recombinant envelope gene which is highly related to the envelope genes found in MCF type-C viruses. This SFFV gp70 migrates as a discrete 70,000 dalton peak on a guanidine hydrochloride-agarose column. The remainder of the coding capacity of the SFFV genome can be accounted for by a greater than 100,000 dalton polyprotein with RLV p12 reactivity. Studies to determine the relationship of either of these proteins to the leukemogenic potential of SFFV are planned.